Sight

If you've ever been in the middle of the ocean under very optimal conditions, you might've been unable to differentiate the sky from the ocean. This is due to the weather being so clear, that you could actually view the true shape of the planet.

Earth is obviously a sphere, however there seems to be a huge misunderstanding. Humanity lives on the inner side of the sphere. Were they to fly straight up, they would end up on the other side of the planet. The reason this doesn't happen, is due to special circumstances.